• / ] _caJ21_gcW211702/2_/9/02 14:24 Art: At'dole Input-lst disk, 2nd CW-trm] i i Oeoc/aumica ¢t Colimo¢/= 'mi¢l _ Vol. 66. No. I 7, pp. 000, _0012 Copynpt O 2002Br,ev_¢ Sclmm Lid ..,y Pergamon _16-7o37_o_$22.00 ÷ .o0 Primed|n_ USA. All ngh_ Pn s0016.7o37(o2_-7 The lAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts J T. W^_ _*_ and G. W. K_a_.M_VN Institute of Gee,physics mid planetary I'hy_lcr_.Univcl"slty of CalU'OITItlLLos Angeles. CA 90095-1567. USA _Ot,_ _ (Recetve,I A,_as_ 7. 2001; a_cepted mrev, sed form January 21. 2002) A__A_WL present new data tt_r mm meteorites that are members of group lAB or are closely related to this lar'[lxe g'r_up._and we have aiNt_ reevaluated some ot our earlier data for these irons. At.'_c':=_ LS_..c,-'.,.z _. __a_t_o d_stln_u_:,n lAB and IIICD irons on the basts of their positions on element-Ni diagrams._we tiadhthat plotting the new and rcvl_d data yields six sets of compact fields on element-Au diagrams, each set corresponding t,_ a _.omposmonal group. The largest set includes the majority (,,.,70) of irons previously designated IA: v,c christened this _t the lAB maan group. The _maining live sets we designate "subgroups" w_thtn the iAB complex Three of these subgroups have Au contents similar to the main group, and form parallel trcntl_ on most elcment-Ni diagrams. The groups originally designated IIIC and IIID are two of these subgroups: they are now well resolved from each other and from the mum group. The other Iow-Au subgroup has Ni cont:nL_ lust above the mum group. Two other lAB subgroups have appreciably higher Au contents than the mum group and show weaker compositional links to it. We have named these live subgroups on the basis hi thor Au and Ni ccmtents. The three subgroups having Au contents similar to the main group arc the low-Au (I.I _uhgroups. the two others the high-Au (H) subgroups. The Ni contents are designated hi gh (H). medium (M _._r low (I, 1.Thus the old group I[ID is now the sLH subgroup, the old group IIIC is the sl.M subgroup. In addition, c_ght irons a_igned to two grouplets plot between sl.L and sLM on most element-Au diagrams. A large number 127) of related irons plot outside these compact fields but nonetheless appear to be sulliciently related to also be included in the lAB complex. Many of these irons contain coarse silicates having similar properties. Most are roughly chondritic in composition: the malic silicates show evidence of reduction during metamorphism. In each case the silicate ()-isotopic composition ts within the carbonaceous chondrite range ( AlTO <- --t)'3e_°°}" In all but four cases these are within the so-called lAB range. -_0 _- A_70 _- -o.68ch_" Fine silicates appear to be ubiquitous in the main group and low-Au subgroups: this requires that viscosities in the parental melt reached high values before buoyancy could separate the_. The well-delined mare-group trends on element-Au diagrams provide constraints for evaluating possible models: we lind the evidence to he most consistent with a crystal segregation model in which solid and melt are es_nttally at equilibrium. T_ac mum arguments against the mare group having formed by fractional crystallization arc: a} the small range m Ir. and h) the evidence for rap_d crystallizauon and a high cooling rate through the 3,-iron stability Iicld. '['hc cwdence lor the latter are the small sizes of the 7-iron crystals parental to the Widmanstatten pattern and the limited thermal effects recorded in the silicates (including retention of albitic plagioclase and abundant pnmordial rare gasesl. In contrast- crystal segregation in a cooling metallic melt (and related processes such .a._incomplete melting and melt migration_ can produce the observed trends in the mare group. We infer that this melt was formed by impact heating on a porous chondritic body. and that the melt was initially hotter than the combined mix of silicates and metal in the local region; the melt cooled rapidly by heat conduction into the cooler surroundings (mainly silicatesL We suggest that the close compositional relationships between the main group and the low-Au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids. Copyright © 2002 Elsevier Science Ltd the origin of LAB. the other large group. Some (Wasson et al.. 1. INTRODUCTION 1980; Choi et al.. 1995) concluded that it is a nonmagmatic group formed as impact generated melts with only minor solid/ There arc four large groups of iron meteorites. All re,arch- liquid partitioning effects superposed. Others (Kracher. 1982, ers agree that three of these (IIAB. II1AB. a_d IVA} formed by 1985" McCoy et al.. 1993; Benedix et al.. 2000) endor'a,_ efficient fractional crystallization ot a slowly cooling magma models involving the fractional crystallization of magnma. IHaack and Scott. 1993: Wasson and Richardson. 2001). Such Kelly and Latimer (1977) envisioned lAB irons to be aue(_$- large sets of irons that formed by fractional crystallization are sive extractions of partial melts from a chondritic source. designated magmaUC groups. There is disagreement regarding Was,son 11999) and Wasson and Richardson (2001) called attention to the advantages both for taxonomy and for ¢osmo- chemical modeling of plotting data for the magmatic groupa on * Aulhot" to whom c_ ndcnc_ should be addt_$e,d element-Au diagrams, as compared to the traditional ele..-, (j.tw_$oll@uei[l_duL Also at: Department of Earth md Spac_ Sciences and Dclrattment of ment-Ni diagrams. The chief advantage for those groups it that Clgtmatry and Bioel_nmma'Y. I [ rich5/21 -gca/21 -gca/211702/213088d02a Inl0ut- 1st disk. _ J G. w. Kalme.a1¢yn berg et al. (1951) and Lovering et al. (1957). When Wauon the total Au range is much larger, but the relative uncertainties (1970) discussed the high-Ga meteorites, he restrictedgroup I (including sampling effects) are as low or lower than those for to fall within certainrelatively narrow fields on Ge-Ga and Ni. As a result, trends in the magmauc groups are much bett_r Ge-Ni diagrams: his group I extended down to Balfour Do_.s. delined on element-Au than on element-Ni diagrams. In this with 56 _g/g Ga and 194 p.g/g Ge. hut he noted that there were pa_r we show that trends involving nonmagmauc group LAB several additional irons that tell along an extrapolation of and similar irons are also better resolved un clcment-Au dia- fields to Lower concentrations. Wasson (1974) designat_i the grams even though the degree of Au fracuonat,m _ _mall Iow-Ga. low-Ge irons in these extrapolated fields IB irons, and compared to that observed in the magmatic groups, t )ur new designated the irons in the original group [ as IA iron& His data set shows that there are several clo,,cty rotated groups view was that these were densely populated and thinly pOlre- within what we now call the lAB complex lated parts of a single genetic sequence. Our current, mo_e complete data set shows this view to be incorrect m detail 2. ANAI,YTICAL TECHNIQUES. SAMPI,ES. AND A small set of related irons was designated IC by Scott and R ESU I .TS Wasson (1976). This group mainly shares relatively high Ga Wc dttcrmln'tgl I_ elements t12 plum I c_ m racial h',in_lrurncnl'dd and Ge contents with lAB. but does not have the high As. Au. ntuU-on-acllVatlonallalysts IINAAI in r_.:plltJal¢ allaivsts, data Ior t'¢ and Sb contents of the latter group. At this time them is no were use_Jfor lntemal Aorln_lZatl(|O, Ihc pn_..edurCsarc those_El'ben hv reason to believe that these irons are closely related m IAB. and \Vas,,a+n¢t al I IUgt;) cxcepl h_"Iwo minor changes Ihc mean ,ample they will not be discussed further in this paper. thickness is now 10 instead ol _ 2 ms. and wc .,,w apply ,mall Groups 111C and IIID were first described by Wasaon and _generallv tn the range 095 t1_ 1051 samplc-+'P -`_-Illt- t_)ITL'ClIOIlS[tl irl+dl_ the NI values 111the lil_l t:ounl a._rec l_.-tlCrwltla lht_st., lr,_m the Schaudy (1971). who noted that the "groups may be re_ to third and fourth C()UnLS(which _ cOlTCLlcdIt+ make I'¢ + "gt = t_l_ each other." Scott (1972) combined the two groups and Scott mglgl. We then +hlx_ a ctlrrm.'lllm lactor Ior the,,¢c_ndi:l=unl ii1= is and Bild ( 19741 noted that several properties of IIICD members iiItgrlnedlatC between that h_r the tir_t and the In¢_,n ,.¢,rrccilon'_ in the third and lourth cllunts. In most cases, cl+ncen_allit, IFIs _1 ( i¢ w,.¢r¢ alSO paralleled those of lAB irons. Scott and Wasson (1975) agreed dctcrmmcd on olber samples by radlt_.'hemtcal ntulrtm actis all(in anal- that the combined i[ICD set was a single group. ysis IRNAAL Was.son et al. (1980) extrapolated IIICD elem_at-Ni tnmds Alll_u2h the INAA data were Satired q,_¢r twH-plus dcc'ad_. down to low Ni concentrations, reclassifying several Iow-Ni slgnihcant improvements in the qualily wcm at-hlcvcd stailJng in ItJg6 lAB irons as IIICD on the basis of Ir.